1. Field of the Invention:
This invention relates to improvements in a control apparatus for a hydraulic motor mainly used as a drive motor for running of a construction vehicle driven by hydraulic power.
2. Description of the Prior Art:
As for drive motors for running of hydraulically driven type construction vehicles such as gyratory excavators, hydraulic motors of variable displacement type have so far been used to enable the vehicle speed to be changed readily. The hydraulic motors of the kind specified have been arranged so that the rotational speed thereof may be controlled by changing the inclination of the inclined shaft or swash plate thereof.
To change the inclination angle of the swash plate, a displacement control cylinder contained in the hydraulic motor is actuated. To achieve this purpose, there have been employed two systems, one of which is of a self-pressure controlled system as described in the Japanese Utility Model Publication No. 59-7647 wherein the pressure to drive the hydraulic motor is supplied directly in the displacement control cylinder, and the other of which is of a pilot pressure controlled system wherein the pressurized fluid from an external pressurized fluid supply source is supplied into the displacement control cylinder.
The control apparatus of the self-pressure controlled system is advantageous in that the number of external conduits attached thereto and the configurational dimension thereof can be reduced, but is disadvantageous in that the fluid pressure for driving the hydraulic motor is required to be kept comparatively high. For example, when a vehicle equipped with a self-pressure controlled type control apparatus for hydraulic motor is running down a slope, the number of revolutions of the engine is reduced and the amount of pressurized fluid supplied from a pressurized fluid supply source into the hydraulic motor is comparatively small. Consequently, the hydraulic motor is driven reversely by the dead load of the vehicle thus causing a pumping effect. As a result, the fluid pressure in the circuit connected between the pressurized fluid supply source and the hydraulic motor is reduced. Whilst, the fluid pressure in the circuit between the delivery side of the hydraulic motor and the counterbalancing valve which is installed in the hydraulic brake valve section is maintained comparatively higher than that in the supply side, because of a restriction in the amount of fluid discharged from the hydraulic motor by the action of the counterbalancing valve itself. If, under such condition, the fluid under pressure supplied into the displacement control cylinder for the hydraulic motor is taken out from the circuit between the pressurized fluid supply source and the hydraulic motor, the pressure in this circuit is so low that it becomes impossible to make controls by the displacement control cylinder by operating the selector valve when the vehicle is running down a slope, thus causing failure in control of the displacement of the hydraulic motor.
Whilst, the pilot pressure controlled type control apparatus for hydraulic motor in which the pressurized fluid from an external pressurized fluid supply source is utilized has no such disadvantage as mentioned above, but is disadvantageous in that it is required to increase the number of external conduits attached thereto, and also increase in the configurational dimension thereof is unavoidable.
The conventional control apparatuses for hydraulic motors are further disadvantageous in that, since a manually operable selector valve is employed to control the displacement control cylinder contained in the hydraulic motor, troublesome operation is required for the operator, and the operating performance thereof becomes low.